Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753375AbbDJCnv (ORCPT ); Thu, 9 Apr 2015 22:43:51 -0400 Received: from mail-vn0-f50.google.com ([209.85.216.50]:42159 "EHLO mail-vn0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751938AbbDJCnu (ORCPT ); Thu, 9 Apr 2015 22:43:50 -0400 MIME-Version: 1.0 In-Reply-To: <1428611797.12911.18.camel@j-VirtualBox> References: <1428521960-5268-1-git-send-email-jason.low2@hp.com> <1428521960-5268-3-git-send-email-jason.low2@hp.com> <20150409053725.GB13871@gmail.com> <1428561611.3506.78.camel@j-VirtualBox> <20150409075311.GA4645@gmail.com> <20150409175652.GI6464@linux.vnet.ibm.com> <1428611797.12911.18.camel@j-VirtualBox> Date: Thu, 9 Apr 2015 22:43:49 -0400 Message-ID: Subject: Re: [PATCH 2/2] locking/rwsem: Use a return variable in rwsem_spin_on_owner() From: Andev To: Jason Low Cc: Linus Torvalds , Paul McKenney , Ingo Molnar , Peter Zijlstra , Davidlohr Bueso , Tim Chen , Thomas Gleixner , Aswin Chandramouleeswaran , LKML Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2747 Lines: 78 On Thu, Apr 9, 2015 at 4:36 PM, Jason Low wrote: > On Thu, 2015-04-09 at 11:16 -0700, Linus Torvalds wrote: >> On Thu, Apr 9, 2015 at 11:08 AM, Linus Torvalds >> wrote: >> > >> > The pointer is a known-safe kernel pointer - it's just that it was >> > "known safe" a few instructions ago, and might be rcu-free'd at any >> > time. >> >> Actually, we could even do something like this: >> >> static inline int sem_owner_on_cpu(struct semaphore *sem, struct >> task_struct *owner) >> { >> int on_cpu; >> >> #ifdef CONFIG_DEBUG_PAGEALLOC >> rcu_read_lock(); >> #endif >> on_cpu = sem->owner == owner && owner->on_cpu; >> #ifdef CONFIG_DEBUG_PAGEALLOC >> rcu_read_unlock(); >> #endif >> return on_cpu; >> } >> >> because we really don't need to hold the RCU lock over the whole loop, >> we just need to validate that the semaphore owner still matches, and >> if so, check that it's on_cpu. >> >> And if CONFIG_DEBUG_PAGEALLOC is set, we don't care about performance >> *at*all*. We will have worse performance problems than doing some RCU >> read-locking inside the loop. >> >> And if CONFIG_DEBUG_PAGEALLOC isn't set, we don't really care about >> locking, since at worst we just access stale memory for one iteration. >> >> Hmm. It's not pretty, but neither is the current "let's just take a >> rcu lock that we don't really need over a loop that doesn't have very >> strict bounding". > > So then something like the following (for rwsem)? > > We can also run some tests to see how the worst case "access > stale memory for one iteration" to the heuristic can have an affect on > performance, though that probably wouldn't be much of an issue in > practice. > > --- > kernel/locking/rwsem-xadd.c | 43 ++++++++++++++++++++++++++++--------------- > 1 files changed, 28 insertions(+), 15 deletions(-) > > diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c > index 3417d01..870c574 100644 > --- a/kernel/locking/rwsem-xadd.c > +++ b/kernel/locking/rwsem-xadd.c > @@ -295,6 +295,31 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem) > } > } > > +static inline bool owner_running(struct rw_semaphore *sem, struct task_struct *owner) > +{ > + bool ret; > + > +#ifdef CONFIG_DEBUG_PAGEALLOC > + rcu_read_lock(); > +#endif Please use 'if (IS_ENABLED(CONFIG_DEBUG_PAGEALLOC)) {}' here. Makes code much readable IMHO. -- Andev -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/